In the field of optical information processing and optical measurement, the present invention relates to an apparatus which uses coherent light beams to automatically perform optical correlation processing on two-dimensional images obtained from image pick-up devices such as CCD cameras.
In recent years, the method and apparatus for pattern recognition which uses coherent light to perform correlation processing has been studied and researched. Most of the researched methods include procedures involving optical Fourier transform correlation wherein input images are optically Fourier transformed and filtered by an optical filter produced by a predetermined procedure. Thereafter the resultant is again officially Fourier transformed to obtain correlation of the input images and image information within the optical filter. The optical Fourier transform correlation apparatus include a matched filter-type optical correlator for filtering Fourier transformed images among the input images using a Fourier transform hologram of a reference image as the optical filter and a joint transform optical correlator for optically Fourier retransforming a joint Fourier transform hologram that is produced from joint Fourier transform images by Fourier transforming simultaneously both the input image and the reference image. The matched filter-type optical correlator, which has a large SN ratio in spite of its difficult alignment, is suitable for an accurate optical pattern recognition. The joint transform optical correlator, which is capable of completing a correlation by only one process with an extremely easier adjustment, is suitable for real-time optical pattern recognition.
Recently, the conventional optical Fourier transform correlation method has been considerably upgraded in its SN ratio by binarizing the optical filter. In addition, liquid crystal type televisions, light addressed type liquid crystal spatial light modulators, and various kinds of photorefractive crystal and like materials have been used as a recording medium of the optical filter instead of the conventional silver photographic plate, and various intensive studies have been undertaken for real-time optical pattern recognition.
However, optical pattern recognition methods other than the conventional optical Fourier transform correlations produce problems in that, when the input image is rotated or its size is changed relative to the reference image recorded in the optical filter, the correlation output is suddenly lowered to substantially degrade the optical pattern recognition.
To solve such drawbacks, there have been efforts made; namely, using as an optical filter the hologram produced at one processing by multiplexing the reference image having various rotating directions and sizes; and employing the optical filter obtained by printing on the photographing plate a synthesized discriminant function that is a computer generated hologram (CGH) capable of hardly reducing a correlation output even when rotation or size change arise in the computer. But, these methods are not suitable for real-time optical pattern recognition because the correlation output is obtained only for a limited variation value of rotation or size change and simultaneously, the formation of the optical filter comes complicated.
Also, the effort is made for the optical Fourier transform correlation using both of the input image and the reference image so that the images are optically coordinate converted into images whose rotation or size change substantially do not occur even when the input image rotates or its size changes. For such coordinate conversion, there are well known a polar coordinate conversion, 1nr-.theta. conversion, and Hough transform and the like, those of which are substantially based on the polar coordinate conversion, where problems arise Sn periodical large change of an output relative to a change of rotation angle due to having many values for the angle component, in addition, complication occurs in producing the computer generated hologram for the coordinate conversion. This coordinate conversion must be performed as an initial stage of processing in the optical Fourier transform correlator, thus tasks are required to produce the computer generated hologram with an extremely higher diffraction efficiency.